1. Field of the Invention
The invention relates generally to adhesive blends and composite structures and, more particularly, the invention relates to extrudable adhesive blends consisting essentially of a modified impact copolymer and a graft reaction product of a propylene polymer and a carboxylic acid or derivative thereof and composite structures incorporating such adhesive blends.
2. Description of Related Technology
Polyolefin blends that include polypropylene-based graft copolymers are useful as adhesives for adhesion to one or more .polypropylene substrates. Typically, in the past, such adhesive blends included a graft copolymer such as one having a polypropylene homopolymer backbone grafted with an acid anhydride such as maleic anhydride blended with a propylene homopolymer or an ethylene/propylene copolymer and mechanically blended to a third component for improving adhesion performance. The third component was often a hydrocarbon elastomer or rubber such as EPDM rubber or EPR rubber.
Although the mechanical addition of an elastomer to a blend may improve its adhesion, such an addition is not always satisfactory because the elastomer typically lowers the softening point of the composition and affects the stiffness and moisture barrier characteristics thereof. Therefore, the addition of an elastomer is undesirable when high temperature stability is required, for example in composite structure applications where the adhesive is bonded to an ethylene vinyl alcohol copolymer substrate. Furthermore, the addition of an elastomer to an adhesive blend adds to the cost of the blend and may cause material handling problems. Also, the presence of elastomers in such blends may result in undesirably high contents of extractable materials which may render the blends unsuitable for use in food packaging and other applications.
Other, substantially elastomer-free adhesive blends are available. However, these blends typically contain a third component other than an elastomer. For example, one prior adhesive blend suitable for use in food packaging that provides adhesion between a polypropylene substrate and a polar or other gas barrier material includes a propylene/ethylene copolymer, a graft copolymer, and a high molecular weight low density polyethylene (HMW LDPE). Although a blend containing HMW LDPE shows an improvement over elastomer-addition blends with respect to food packaging, the addition of any type of third component adds to the expense of an adhesive blend.